Various distance measuring procedures are known for collision warning devices also known as parking aids. German Patent Application No. 40 23 538 describes a collision warning device based on ultrasound in which at least two ultrasound sensors are mounted at a specified distance from each other. In this procedure, the delay times between the time when an ultrasound signal is emitted and a reflected ultrasound signal is received from the same and from the other ultrasound sensors are measured and evaluated.
The measuring range of ultrasound sensors is limited by the beam angle, the size of the obstacle, its reflective properties and by the detection threshold of the sensor. This means, for one thing, that a "dead angle" results for each of the ultrasound sensors. In addition, within the recording angle, no distance measurement will be carried out within the immediate vicinity because of the decay period of the sensors.
In contrast, capacitive sensors have the advantage that measurement is possible up to the shortest distances. In addition, if the sensor is designed appropriately, a measurement without voids can be carried out over the entire vehicle width. However, these advantages are countered by the disadvantage that, due to the various materials, sizes and shapes of the possible obstacles, no firm connection exists between the distance and the measurable capacitance in each case, i.e. only relative distance values are determined.
British Patent No. 2,266,397 describes a motor vehicle parking aid in which a distance to a possible obstacle is monitored using an ultrasound sensor. Since the ultrasound sensor cannot monitor all areas, in particular not the area in the proximity of the vehicle, a second measuring system is provided. This measuring system is configured as a capacitive system and detects obstacle at a minimum distance from the vehicle. The publication, Kramar E., "Funksysteme fur Ortung und Navigation" (Radio systems for location and navigation), 1973, Verl. Berliner Union GmbH, Stuttgart, Germany, pp. 28 and 29, discloses a phase difference measuring process for measuring the distance between two radiation sources A and B, in which radiation sources A and B transmit in-phase HF signals, received at the receiving station with a certain phase difference.
The object of the present invention is to provide a procedure for non-contact distance measurement, in particular for measurement of the distances from obstacles in the immediate vicinity of motor vehicles, in which measurement of short distance values is also possible.